The Taliban have condemned a "horrific" video that apparently shows fighters from the Islamic State group blowing up bound and blindfolded Afghan prisoners with explosives.

The Taliban have condemned a "horrific" video that apparently shows fighters from the Islamic State group blowing up bound and blindfolded Afghan prisoners with explosives, spotlighting the growing rivalry between the militant networks.

IS insurgents have been making gradual inroads into Afghanistan, challenging the Taliban on their home turf at a time when an increasingly bitter power transition is roiling the Afghan militant movement.

The Taliban have themselves often been accused of savagery during their 14-year insurgency against the US-backed Afghan government but they were blunt in their condemnation of the new IS video.

"A horrific video... (shows) kidnappers who associate themselves with Daesh (IS) brutally martyring several white-bearded tribal elders and villagers with explosives," the Taliban said in a statement Tuesday.

The video, more than four minutes long, appeared on jihadi social media forums on Sunday and contained commentary in Arabic and Pashto.Apparently shot in Afghanistan's restive east against the backdrop of hilly grasslands enveloped with fog, it describes the prisoners as "apostates" aligned with the Taliban or the Afghan government.

But the Taliban described them as "innocent civilians".

"This un-Islamic act... can never be justified," their statement said.

"This offence and other such brutal actions by a few irresponsible ignorant individuals under the guise of Islam and Muslims are intolerable."

The Taliban are blamed for the vast majority of civilian casualties in Afghanistan as the country battles a growing wave of insurgent attacks following the announcement of the death of longtime Taliban leader Mullah Omar.

Even before news of his death the group suffered a string of defections to IS, with some insurgents voicing disaffection with the man described as a "ghost leader" -- he had not been seen in public since the Taliban were toppled from power in 2001.

'Taliban's number one enemy'

The announcement of Omar's death, observers say, could be a very effective recruitment tool for IS, potentially helping it lure more Taliban turncoats in a region where it is still struggling to gain ground.

The Taliban warned IS recently against expanding in the region, but this has not stopped some fighters swearing allegiance to IS chief Abu Bakr al-Baghdadi.

"Right now the Taliban's number one enemy is IS," Kabul-based military analyst Atiqullah Amarkhil told AFP.

"With (Tuesday's) condemnation the Taliban want to project themselves as a legitimate group waging an Islamic war -- and IS are a foreign phenomenon trying to weaken Islam. But the Taliban's crimes are not hidden from the Afghan people."

Experts say the gravitational pull of IS, renowned among jihadists for establishing an Islamic "caliphate" across a swathe of Syria and Iraq, is only likely to grow amid a bitter power struggle within the Taliban.

Some top leaders including Omar's son and brother have refused to pledge allegiance to new leader Mullah Akhtar Mansour, saying the process to select him was rushed and even biased.

Tayeb Agha, the head of the Taliban's Qatar political office set up in 2013 to ease talks with Kabul, resigned last week in protest at Mansour's appointment and two more members of the office followed suit.